Recently, a renewed interest in thin-film magnetic random access memories (MRAM) has been sparked by the potential application of MRAM to both nonvolatile and volatile memories. FIG. 1 depicts a portion of a conventional MRAM 1. The conventional MRAM includes conventional orthogonal conductor lines 10 and 12, conventional magnetic storage cell 11 and conventional transistor 13. The conventional MRAM 1 utilizes a conventional magnetic tunneling junction (MTJ) stack 11 as a memory cell. Use of a conventional MTJ stack 11 makes it possible to design an MRAM cell with high integration density, high speed, low read power, and soft error rate (SER) immunity. The conductive lines 10 and 12 are used for writing data into the magnetic storage device 11. The MTJ stack 11 is located on the intersection of and between 10 and 12. Conventional conductive line 10 and line 12 are referred to as the conventional word line 10 and the conventional bit line 12, respectively. The names, however, are interchangeable. Other names, such as row line, column line, digit line, and data line, may also be used.
The conventional MTJ 11 stack primarily includes the free layer 1103 with the changeable magnetic vector (not explicitly shown), the pinned layer 1101 with the fixed magnetic vector (not explicitly shown), and the insulator 1102 in between the two magnetic layers 1101 and 1103. The insulator 1102 typically has a thickness that is low enough to allow tunneling of charge carriers between the magnetic layers 1101 and 1103. Layer 1100 is usually a composite of seed layers and an anti-ferromagnetic layer that is strongly coupled to the pinned magnetic layer. Layer 1104 is a nonmagnetic capping layer, which protects the underlying layers 1100, 1101, 1102, and 1103.
Data is stored in the conventional MTJ stack 11 by applying a magnetic field to the conventional MTJ stack 11. The applied magnetic field has a direction chosen to move the changeable magnetic vector of the free layer 1103 to a selected orientation. During writing, the electrical current I1 flowing in the conventional bit line 12 and I2 flowing in the conventional word line 10 yield two magnetic fields on the free layer 1103. In response to the magnetic fields generated by the currents I1 and I2, the magnetic vector in free layer 1103 is oriented in a particular, stable direction. This direction depends on the direction and amplitude of I1 and I2 and the properties and shape of the free layer 1103. Generally, writing a zero (0) requires the direction of either I1 or I2 to be different than when writing a one (1). Typically, the aligned orientation can be designated a logic 1 or 0, while the misaligned orientation is the opposite, i.e., a logic 0 or 1, respectively.
Stored data is read or sensed by passing a current through the conventional MTJ cell from one magnetic layer to the other. During reading, the conventional transistor 13 is turned on and a small tunneling current flows through the conventional MTJ cell. The amount of the current flowing through the conventional MTJ cell 11 or the voltage drop across the conventional MTJ cell 11 is measured to determine the state of the memory cell. In some designs, the conventional transistor 13 is replaced by a diode, or completely omitted, with the conventional MTJ cell 11 in direct contact with the conventional word line 10.
Although the above conventional MTJ cell 11 can be written using the conventional word line 10 and conventional bit line 12, one of ordinary skill in the art will readily recognize that the amplitude of I1 or I2 is in the order of several milli-Amperes for most designs. Therefore, one of ordinary skill in the art will also recognize that a smaller writing current is desired for many memory applications.
Furthermore, to be competitive with other types of memory, the density and capacity of an MRAM chip embodying technology such as the conventional MRAM 1 should be comparable with that of DRAM, FLASH or SRAM products. For state-of-the-art technology, the size of an MRAM cell is in the submicron range. The lateral size of the MTJ stack 11 is even smaller, in the deep submicron range. Moreover, as memory densities increase, the lateral size of the MTJ stack 11 is further reduced. The small size of the MTJ stack 11 leads to problems in the performance of the conventional MRAM 1.
As the lateral dimensions of the MRAM cell and MTJ stack 11 are reduced, the volume of each of the magnetic layers 1101 and 1103 in the conventional MTJ stack 11 is reduced. The reduction in volume of the magnetic layers 1101 and 1103 leads to the possibility of “super-paramagnetic” behavior. For a layer exhibiting super-paramagnetic behavior, thermal fluctuations can cause the magnetic moment of the layer to spontaneously rotate if the magnetic anisotropy of the layer is not sufficiently large. The magnetic anisotropy of a layer, or other magnetic entity, is proportional to its volume. Consequently, layers 1101 and 1103 are more likely to exhibit super-paramagnetic behavior for conventional MTJ stacks 11 having smaller lateral dimensions.
Conventional MTJ stacks 11 having smaller lateral dimensions may also have increased magnetic interactions between the magnetic layers 1101 and 1103. The stray magnetic fields at the edges of the MTJ cell are generated by the magnetic poles at the edges of the ferromagnetic layers 1101 and 1103 of the conventional MTJ stack 11. These stray magnetic fields increase in magnitude for smaller conventional MTJ stacks 11. Stray magnetic fields can lead to large magnetic interactions between the pinned layer 1101 and the free layer 1103 within a single MTJ storage cell. Due to magnetic interactions between the ferromagnetic layers 1101 and 1103 in a single conventional MTJ cell, the state of the MTJ cell in which the magnetic moments of the ferromagnetic layers 1101 and 1103 are antiparallel is more energetically stable than the state in which the moments of the layers 1101 and 1103 are parallel. This asymmetry in the stability of the conventional MTJ stack 111 leads to asymmetries in the magnetic switching between the parallel and antiparallel states of the MTJ cell. Consequently, it becomes difficult to operate the MTJ cell and, therefore, the conventional MRAM 1. The stray magnetic fields can also lead to magnetic interactions between the ferromagnetic layers 1101 and 1103 of one conventional MTJ stack 111 and the ferromagnetic layers (not shown) of neighboring MTJ storage cells. In this case, the magnetic switching field of a given MTJ cell depends on the magnetic state of its neighboring MTJ cells. Consequently, the margin of write operations of the memory array is reduced. Eventually, the conventional MRAM 1 becomes inoperable. Unless these magnetostatic interactions can be mitigated, the smallest size of the MTJ cells and thus the highest density of the MTJ MRAM are limited.
The magnetostatic fields emanating from the exchange-biased pinned layer 1101 can be greatly reduced by replacing the conventional pinned layer 1101 with a sandwich of two ferromagnetic films antiferromagnetically coupled to one another and separated by a thin antiferromagnetic coupling film. The antiferromagnetically coupled films together with the antiferromagnetic coupling film thus form a synthetic pinned layer. Such a system is described in U.S. Pat. No. 5,841,692. In the synthetic pinned layer, the magnetic moments of the two ferromagnetic films in the pinned layer are aligned antiparallel. Consequently, the net magnetic moment of the synthetic pinned layer is reduced compared to a pinned layer comprised of a single ferromagnetic layer. Because the strength of the magnetostatic field from a ferromagnetic layer is proportional to the net magnetic moment of the layer, the magnetostatic field from the synthetic pinned layer is less than that of a pinned layer comprised of a single ferromagnetic layer.
It is also possible to reduce the strength of the magnetostatic fields emanating from the edges of the free layer 1103 of the conventional MTJ 11 by forming a synthetic free layer from a sandwich of two antiferromagnetically coupled ferromagnetic films which are separated by an antiferromagnetic coupling film. However, the synthetic free layer may have several disadvantages. The antiferromagnetic coupling film is extremely thin. Consequently, the thermal stability of the antiferromagnetically coupled ferromagnetic films may not be adequate for the required wafer processing steps to which the MTJ materials will be subjected. The antiferromagnetic coupling may, therefore, be broken. In addition, the magnetic properties of the synthetic free layer may be inferior to that of the individual ferromagnetic films because of an incomplete antiferromagnetic coupling between the ferromagnetic films if the films. The antiferromagnetic coupling may be incomplete for several reasons, for example, the use of ferromagnetic films that are not extremely smooth. When fabricating a conventional MTJ stack 11, it is usually preferable to first deposit the metallic antiferromagnetic layer included in the layers 1100 and the pinned layer 1101 to which it is exchange-biased to achieve optimal exchange biasing. This order is preferred because metallic layers grown on top of the insulator 1102, which is usually formed from an amorphous layer of Al2O3, may be rougher than the layers formed beneath the insulator 1102. Typically metal layers do not “wet” oxide layers so that thin metal layers deposited on oxide layers are comprised of numerous islands of varying sizes and heights. Such metal layers are necessarily rough. Moreover, such layers will have a poor crystallographic texture. For these reasons, it is very difficult not only to prepare very thin ferromagnetic free layers 1103 with good magnetic properties but also antiferromagnetically coupled free layers with appropriate magnetic properties.
U.S. Pat. No. 6,166,948 discloses one conventional method for addressing this problem. The MTJ cell disclosed in the patent has a multilayer free layer including two ferromagnetic films that are magnetostatically coupled antiparallel to one another by their respective dipole fields. The magnetostatic, or dipolar, coupling of the two ferromagnetic films occurs across a nonferromagnetic spacer layer. The nonferromagnetic spacer layer is selected to prevent exchange coupling between the two ferromagnetic films. The magnetic moments of the two ferromagnetic films are antiparallel to each other so that the multilayer free layer has a reduced net magnetic moment. The reduced net magnetic moment of the multilayer free layer reduces the magnetostatic coupling between the multilayer free layer and the pinned layer in the MTJ cell. The reduced magnetic moment of the multilayer free layer also reduces the magnetostatic coupling between adjacent MTJ cells in the array. However, based on the principles disclosed in the patent, the two layers of ferromagnetic films have very different properties. For example, one film is very thick, has a low magnetization and close-to-zero coercivity. The other film is thin, has high magnetization and high coercivity. Under these conditions, the moment of the free layer of the MTJ device can be reduced by more than forty percent but still far from being cancelled completely. The interaction field between cells is still about sixty of that in a single ferromagnetic layer free layer case. Additionally, shape anisotropy may make it impossible to achieve close to zero coercivity with either of the two ferromagnetic layers. Consequently, this scheme is very difficult to implement.
Accordingly, what is needed is a magnetic memory in which the moment of the free layer of the MTJ can be completely cancelled or reduced, thereby reducing or eliminating magnetic interactions between magnetic layers within a cell and between adjacent cells. In addition, it would also be desirable for the MTJ cells to be protected against stray magnetic field and to have improved write efficiency. The present invention addresses such a need.